doc: update SEA source link

The code was moved in 3803b02 so the current source link is broken
in our docs.

PR-URL: nodejs#48080
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/single-executable-applications.md

@@ -4,7 +4,7 @@
 
 > Stability: 1 - Experimental: This feature is being designed and will change.
 
-<!-- source_link=lib/internal/main/single_executable_application.js -->
+<!-- source_link=src/node_sea.cc -->
 
 This feature allows the distribution of a Node.js application conveniently to a
 system that does not have Node.js installed.